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Nanoparticles are microscopic particles with a diameter range between 1 and 100 nm. These particles possess physical properties such as high conductance, uniformity and special optical properties which make them ideal for use in biology and materials science. Nanoparticles have become an area of intense scientific research due to the broad range of potential applications in the optical, biomedical and electronic fields.
Nanoparticles can be in the form of nanocrystals, nanopowders or nanoclusters. The particles act as a bridge between bulk materials and atomic or molecular structures. They exhibit several special properties corresponding to the bulk material; for instance, the bending of bulk copper occurs due to the movement of copper atoms/clusters at the 50 nm scale. Copper nanoparticles that measure less than 50 nm are known as super-hard materials which do not display the same ductility and malleability as bulk copper.
There are different types of nanomaterials such as carbon nanotubes, fullerenes (including buckyballs), dendrimers, fine and ultrafine particulates in the air, titanium dioxide nanoparticles, quantum dots and nanocrystals, silver nanowire, silver nanoparticles, and other nano-sized particles.
The properties of materials alter as their size reaches the nanoscale and as the percentage of atoms at the material’s surface becomes significant. However, the change in properties is not always desirable. Ferroelectric materials that measure less than 10 nm can change their magnetization direction using room temperature thermal energy, making them ineffective for memory storage.
Nanoparticles frequently have unpredicted visible properties as they are small enough to lock up their electrons and generate quantum effects. For instance, gold nanoparticles look deep red to black in solution.
Nanoparticles have a very high surface area to volume ratio which provides a great driving force for diffusion, particularly at high temperatures. Sintering can be performed at lower temperatures across shorter time scales compared to larger particles, and the large surface area to volume ratio decreases the initial melting temperature of nanoparticles.
In recent years, nanoparticles have emerged as key players in modern medicine. They are being tested for use in numerous clinical applications ranging from carriers for drug and gene delivery to tumors, to contrast agents in imaging. Polymeric micelle nanoparticles are used to deliver drugs to tumors, and carbon nanoparticles called nanodiamonds are being tested for use in other medical applications. For example, protein molecules can be attached to nanodiamonds to increase bone growth around joint or dental implants.
Chemotherapy drugs attached to nanodiamonds are being tested for brain tumor treatment and some researchers are looking to use the same to treat leukemia.
A special dye is used in photodynamic cancer therapy to create atomic oxygen, which is cytotoxic. Most of this dye is absorbed by cancer cells when compared to the healthy tissue. Therefore, only the cancer cells are destroyed which are subsequently exposed to laser radiation. A major drawback of this therapy is that the residual dye molecules spread to the eyes and skin, making the patient highly sensitive to daylight exposure.
To prevent this side effect, the hydrophobic version of the dye molecule is encapsulated in a porous nanoparticle. The dye remains trapped within the Ormosil nanoparticle and does not migrate to other parts of the body, and the ability to produce oxygen remains. Polymer coated iron oxide nanoparticles are used to break up clusters of bacteria and could facilitate more effective treatment of chronic bacterial infections. In another study, cerium oxide nanoparticles were applied as an antioxidant to remove oxygen-free radicals present in a patient's bloodstream following a traumatic injury.
The nanoparticles absorb the oxygen-free radicals and then discharge the oxygen in a less hazardous state. This frees up the nanoparticles to absorb additional free radicals. Nanoparticles may also be used in inhalable vaccines in the future as the surface change of protein-filled nanoparticles impacts the ability of the nanoparticle to stimulate immune responses.
The natural bone surface in most cases contains features that are approximately 100 nm across. If an artificial bone implant’s surface is smooth, the body will reject it. By creating nano-sized features on the surface of the knee or hip prosthesis, the chances of rejection can be reduced.
The production of osteoblasts can also be simulated and has been demonstrated using ceramic and polymeric materials as well as some metals. This development could help in the design of a robust and long-lasting hip or knee replacement and reduce the chances of the implant becoming loose.
Manufacturing and Materials
Nanotechnology is being used by many manufacturers to develop products with better capabilities or to lower production costs. Researchers at Purdue University have demonstrated a method called laser shock imprinting that produces nanoscale metallic shapes which could potentially give them appealing optical and mechanical properties using a bench-top system. This is a cost-effective system which can produce the shapes on a large scale. A major advantage of this shock-induced forming method is that it creates high-fidelity structures and sharply defined vertical features and corners.
In another study, Northwestern University researchers developed a low-cost, high-resolution nanofabrication tool which employs beam-pen lithography arrays to produce nanoscale structures. This tool enables users to quickly process substrates that are coated with photosensitive materials known as resists and create structures that cover the micro, macro and nanoscales, all in a single experiment.
Materials fabrication and materials innovation are at the core of nanoscale science and engineering. Nanotechnology makes it possible to modify the materials’ key structures at the nanoscale to obtain the desired properties. Through nanotechnology, materials can be made lighter, stronger, more durable, more sieve-like, more reactive and behave better as electrical conductors.
Over 800 commercial products depend on nanoscale processes and materials to make them strong, lightweight, resilient and long-lasting; these include additives in polymer composite materials for tennis rackets, baseball bats, automobile bumpers, motorcycle helmets, power tool housings and luggage. Similarly, the use of nanoscale additives for surface treatments of fabrics makes them resistant to staining, wrinkling and bacterial growth.
Nanoparticles are being used in many applications to improve the environment. For instance, photocatalytic copper tungsten oxide nanoparticles are being tested to break down oil into biodegradable compounds. The nanoparticles are placed in a grid to provide high surface area for the reaction. They can be activated by sunlight and can work in water, making them useful for cleaning up oil spills. Another example is gold nanoparticles embedded in a porous manganese oxide, which is being used as a room temperature catalyst to breakdown volatile organic air pollutants.
Similarly, carbon tetrachloride pollution in groundwater and arsenic present in water wells can be removed using iron nanoparticles.
Energy and Electronics
Researchers are exploring the use of nanotechnology to generate more efficient and cost-effective energy. When sunlight is concentrated on nanoparticles, it creates steam with high energy efficiency.
Similarly, high-efficiency light bulbs can be produced with a nano-engineered polymer matrix in a specific style. These innovative bulbs are shatterproof and have double the efficiency of fluorescence light bulbs.
Attempts are also being made to develop high-efficiency LEDs using an array of nano-sized structures known as plasmonic cavities. Similarly, windmill blades are being developed using epoxy containing carbon nanotubes. These nanotube-filled epoxies help to create stronger yet lightweight blades, which boost the amount of electricity produced by individual windmills.
Nanotube sheets have also been used to create thermocells, which produce electricity when the cell sides are at varying temperatures. For example, these nanotube sheets can be enclosed around a car’s exhaust pipe to produce electricity from heat which is otherwise wasted.
Nanotechnology is employed in many different electronics, communications and computing applications, providing smaller, faster and more portable systems, which can store large amounts of data.
Some further examples of nanoelectronics include cell phone castings, flash memory chips for the iPod nano, antibacterial and antimicrobial coatings on computer hardware such as the keyboard and mouse. Nanotechnology is used in smart cards, printed electronics for Radio Frequency Identification (RFID) and smart packaging, as well as in flexible displays for e-book readers and life-like video games. It is even used in nanoscale transistors that are more powerful, faster and highly energy-efficient. In the future, the entire memory of a computer may be stored on a just one small chip.
Other uses of nanotechnology include next-generation televisions, plasma displays, digital cameras, laptops, quantum computers, magnetic random access memory and organic light-emitting diodes (OLEDs). Nanotechnology is set to redefine many electronic products, processes and applications in future.
Multicolor Optical Coding for Biological Assays
Another application of nanoparticles is proteomics and genomics, which have become a key area of research as they produce an increasing amount of sequence data. Many array technologies currently in use in parallel analysis will reach saturation when the number of array elements surpasses several million.
Single quantum dots of compound semiconductors were effectively used as a substitute of organic dyes in many bio-tagging applications. This concept was enhanced further by combining differently sized dots in an array of fluorescent colors and integrating them into polymeric microbeads. This provided a precise control of quantum dot ratios. The variety of nanoparticles used in this experiment had 6 different colors and 10 intensities, which was adequate to encode more than a million combinations.
Manipulation of Cells and Biomolecules
Functionalized magnetic nanoparticles are being used in several applications, such as cell separation and probing. Since the magnetic nanoparticles explored so far are spherical, they cannot be used to maximum capacity.
Alternative cylindrically-shaped nanoparticles can be developed by applying metal electrodeposition into nanoporous alumina template. Based on the template’s properties, nanocylinder radius can be selected between the ranges of 5 and 500 nm, while their length can be as long as 60 µm. The magnetic and structural properties of separate cylinders can be modified extensively through sequential deposition of different thicknesses of various metals.
As the surface chemistry for functionalization of metal surfaces is properly developed, different ligands can be selectively bound to different segments. For example, carboxyl linkers or porphyrins with thiol were concurrently bound to the nickel or gold segments, respectively, allowing magnetic nanowires to be produced with spatially segregated fluorescent parts. Additionally, due to the large aspect ratios, the residual magnetization of these nanowires can be high, so a weaker magnetic field can be applied to drive them.
It has been revealed that weak external magnetic fields can be used to manipulate the self-assembly of magnetic nanowires in suspension. This could help to control cell assembly in various forms and shapes.
Proteins are a vital part of the cell's structure, language and machinery, therefore a better insight into their functionality is imperative to continue to improve the well-being of humans. Gold nanoparticles are extensively used in immunohistochemistry to detect the interaction between proteins. This method has comparatively limited detection capabilities.
In contrast, surface-enhanced Raman scattering spectroscopy has demonstrated its suitability for detection and identification of single dye molecules. By integrating both of these techniques in a single nanoparticle probe, researchers can greatly improve the multiplexing capabilities of protein probes.
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After years of research and development, commercial products enabled by nanoparticles are set to make a huge impact in various sectors of the global economy, ranging from electronics and healthcare to energy. According to a study published in April 2016, nanoparticles serve as a promising vehicle for clinical gene therapy due to their tunable size, surface, shape and biological behaviors.
Nanobots do not technically exist yet, but when they do, the potential applications in which they could be used include molecular manufacturing and medical nanobots. These would autonomously navigate through the bloodstream, repairing cells, attacking viruses and guarding against infections.
Products like magnetite, hematite and maghemite nanoparticles have promising properties for biomedical applications. Additionally, researchers in Korea and China have reviewed some of the latest studies on the structure, preparation and magnetic properties of iron oxide nanoparticles (IONPs) and their corresponding applications.
Scientists are looking toward nanotechnology and nanoagriculture to find solutions to growing population concerns, including studies on how nano-sized particles can boost crop and livestock productivity. The benefits of nanoagriculture include the potential to monitor plant growth, protect plants, detect plant and animal diseases, boost global food production, improve food quality and reduce waste. Scientists are also eager to apply nanotechnology to resolve water issues around the world and make water safe and pure.
To speed up solar power advancements, scientists are applying nanotechnology to solar energy. Nanoparticles possess the ability to improve the absorption of light, boost the conversion of light to electricity and offer improved thermal storage and transport. With these advantages, nanotechnology has the potential to optimize solar energy efficiency and minimize costs.
In the next two decades, nanotechnology will impact almost every human being on Earth. The potential benefits of nanoparticles are likely to make great strides in the future.
Sources and Further Reading
This article was updated on the 2nd September, 2019.